A toddler whom police had pronounced dead after she fell into the river Thames when her mother fainted while feeding ducks was resuscitated after doctors discovered a very faint heartbeat two hours later, and was last night in a critical condition at the John Radcliffe hospital in Oxford.

The 18-month-old girl, who has not been named, had been airlifted to the hospital and had been thought to be dead when she arrived. A police statement announced her death shortly before midday, but two hours later, a spokesman said that she had come back to life.

The incident happened at around 9.45am at Ferry Lane, Goring, South Oxfordshire, after the child fell from a towpath into the water with her mother and six-year-old sister. A passing boatman jumped into the water and dialled emergency services from a mobile phone.

Sergeant Graham Pink, of Thames Valley police, said: "The child was pronounced dead at the John Radcliffe hospital before a very faint heartbeat was discovered. The child remains in a critical condition. We put a statement out saying the child was dead and we would not have done that unless we had been told she was dead. The baby's mother and her other child, who was also on the river bank, were also taken to hospital."

Neither the mother nor the older child needed hospital treatment.

The scene was cordoned off yesterday as officers, including police divers, investigated. Pink said a man in a boat was believed to have seen the baby in the water and jumped in to help.

Rescuers from Oxfordshire fire and rescue, who went to the scene, said they were told that one adult and two children had been pulled from the river by a passer-by. "The circumstances leading up to the incident are unclear," said a spokesman.

One of the first rescuers to arrive said a boatman had jumped in and managed to get two of the victims out of the water and was rescuing the third when he arrived. "All three of them were in the river and all had swallowed quite a lot of water," he said. "Tragically the little girl, who was only a toddler, had taken on so much water it was feared there was nothing the paramedics could do to revive her. A few minutes after they started giving emergency aid to the two children and the mother, the air ambulance hovered overhead and then landed. The paramedics put the toddler into the helicopter and flew her to hospital in Oxford, but she was pronounced dead on arrival. It is amazing news that a heartbeat has since been found and she is now alive again.

"The mother was conscious and told us that she had passed out while feeding the ducks and fallen into the river, taking the two children with her."

A spokeswoman for South Central ambulance service said: "We were called to a report of a possible drowning, with a female and a child in the water. When we arrived, the toddler was out of the water. She was unconscious and not breathing. We did not declare her dead."

A spokeswoman for the Oxford Radcliffe hospitals NHS trust said she could not comment on police suggestions that medics from the trust first declared the girl dead, then realised she was alive. "A faint heartbeat was discovered as doctors tried to resuscitate her. She is now in a critical condition," she said.

In December 2005, two-week-old Woody Lander, from Leeds, who had been pronounced dead after a heart attack, coughed while his parents were cuddling him half an hour later and was resuscitated.